Reduced # of supported arguments in `_Deferred` conversion operators.
Reduced # of supported arguments in `_Deferred` conversion operators.

Conversion of `_Deferred` to `std::function` and `Deferred` currently
supports up to 12 parameters in function signature. However, this is
unnecessary and is a huge overhead. Most usages require just one
parameter (e.g. when `defer` is used with `Future`). And there are few
usages with two parameters (in `master.cpp` to initialize allocator, and
in `slave.cpp` to install signal handler). This number of parameters is
different from the number of parameters passed to `defer`, but it's
related and can be defined as maximum number of placeholders that can be
passed to `defer`.

Given that `deferred.hpp` is indirectly included in most source files,
it is beneficial to keep this number low. This patch changes maximum
number of parameters to 2.

Review: https://reviews.apache.org/r/64338/

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